introduction to supernovae According to the In any case, no European record of the event has Annals of the Sung Dynasty (the ever been found. Sung-shih), on the first day of the chi-ho Since the appearance nearly a millennium ago of reign period, during the 5th month, on the the Sung “guest star” there have been only two chi-chou, a “guest star” appeared to the south other similar objects seen in our Galaxy. One oc- east of Tian-kuan. curred in 1572 in the constellation Cassiopeia. The guest star was so This was observed by the Danish astronomer Ty- bright that it could be cho Brahe and bears his name. It became bright seen during the day- enough to be visible in full daylight. The other time, and it remained star appeared in the constellation Ophiuchus in so for 23 days. Af- 1604 and was studied by Tycho’s student and col- An ter that, it gradually laborator, Johannes Kepler, though it was seen un- dimmed, finally fading known artist earlier by several other people. Kepler’s star, while may have recreated the from visibility after two years. not as bright as Tycho’s, was still as bright as Jupi- appearance of the Crab Japanese records also mention the ter. Since the appearance of Kepler’s star, no oth- Nebula supernova in star. 1054 on the side of ers have been seen in the Galaxy. a cave wall in Chaco At around the same time, half a This does not mean, however, that no additional Canyon, New Mexico. world away from China in what similar objects have been observed. In 1885 a new we now call Chaco Canyon, in star appeared in the center of the Milky Way’s Northern New Mexico, an ancestor of today’s companion galaxy M31, in the constellation of Hopi and Navajo painted what appears to be a Andromeda. It reached a peak brightness of 6-7th “guest star” into a protected rock overhang. The magnitude, making it easily visible in small tele- star is shown next to the crescent moon, and scopes against the background glow of the galaxy a hand print, perhaps that of the artist, is also itself. The object is important for historical rea- painted on the rock. Though this record is not sons because it was used to argue, incorrectly, that as detailed as that of the Sung astronomers, the the great spiral nebula of Andromeda was a new orientation of the moon and the guest star is star system, like our solar system, in the process of what it would have been on that day when the forming within our own galaxy. The astronomer Sung reported their guest star, strongly suggest- Harlow Shapley, in a famous 1922 debate with ing that the two are the same. Heber Curtis on the nature of the spiral nebulae, Such an impressive object, recorded in dispa- claimed that the appearance of the guest star in rate cultures around the globe, must have been Andromeda was due to a “new sun” just begin- visible in Europe as well as Asia and North ning to turn on. His argument was later shown to America. However, the date given in the Chi- be wrong. Edwin Hubble measured the distance nese annuls, by our modern reckoning, would to the Andromeda nebula and proved it was ex- have been July 4, 1054. At that time Europe- tremely far away and, in fact, an independent sys- ans were in the throes of the Dark Ages, and tem of stars – a galaxy on a par with our own the Norman Invasion was just a few years Milky Way. away. Perhaps they were too occupied Though these guest stars are rare with worldly concerns to mark down events in any given galaxy, the the appearance of a celestial visitor, universe or perhaps whatever record existed has been lost. The Crab Nebula is located just above the star mark- ing the tip of the lower horn of Taurus, the Bull. If you point a telescope toward the patch of sky described in the Chinese records from 1054, just a few degrees north and east of Aldebaran, the “eye” of Taurus, the bull, you will find a faintly glowing cloud. This is the Crab Nebula. It is the remains of a star that exploded some 7000 years ago. The explosion was seen on contains many, many galaxies. With the ad- Earth only 1000 years ago because it was vent of large telescopes in the 1920s and 30s it so distant that its light required 6000 years was soon noticed that guest stars could be seen to reach us; the Sung and Chaco Canyon quite often if one looked at many galaxies. The inhabitants were seeing the explosion 6000 fact that the guest stars were nearly as bright as years after it happened. The Crab Nebula the galaxies in which they occurred meant that is a supernova remnant, the debris from they were enormously energetic. Their great an exploded star. It is still expanding brightness and release of energy prompted the today at more than 1000 km/s, (for astronomer Fritz Zwicky to dub them superno- more about supernova expansion vae, because they appeared similar to, but far see p.11) having slowed from an brighter than, the “novae” seen in our galaxy. initial expansion speed of more than Supernova is the name by which we still call 10,000 km/s. Inside the nebula is them today, though we now know they have the Crab pulsar, the compact nothing in common with novae except a name: remnant of the core of the supernovae are exploding stars, whereas novae exploded star. The pulsar is a are the much smaller explosion of the atmo- highly magnetized, rapidly sphere of a white dwarf star that is acquiring spinning neutron star, a class matter from a nearby binary companion star. of object that is among the (For more about supernova life cycle, see p.8.) most bizarre found in nature. In an ironic twist, recent observations of super- A mere teaspoon of the crab novae similar to the one seen in Andromeda pulsar would weigh in 1885 have allowed us to measure the vast more than a billion size and expansion rate of the universe. To our tons (for more about great surprise, these extremely distant superno- neutron stars see vae indicate that the expansion is accelerating, p.22.) rather than slowing down. These observations In the remainder indicate that approximately 70% of the energy of this education in the universe is something never before unit, you will ex- observed, with properties heretofore only plore the amazing properties imagined in the most speculative of our of supernovae and neutron stars. theories of nature. Far from showing You will also begin to learn that the universe is small, as Shapley ar- about some of the gued, supernovae have shown us that tools scientists use the universe is not only vast, but to understand much stranger than we had them. imagined. why stars explode The stars in the sky seem eternal and unchanging. But that’s an illusion. Like Initially, the star fuses hydrogen into helium. Like all things, stars are born, live out ash in a fire, the helium builds up in the core, but it their lives, and eventually die, doomed does not fuse because helium takes a lot more pres- to fade away. Stars like the Sun, which have sure and heat than hydrogen does to fuse. If the star a relatively low mass, age gracefully and die is massive enough, though, it can ignite helium fu- quietly after billions of years. But massive stars, sion in its core. The helium fuses into carbon, which with more than ten or so times the mass then starts to pile up in the core. In very mas- of the Sun, “do not go gently into sive stars this process repeats again and H that good night, but instead again, fusing lighter elements into rage, rage against the dying of He heavier ones: hydrogen to he- the light”. They explode in lium, helium to carbon, carbon a catastrophic detonation, C to neon, neon to oxygen, oxy- sending their outer layers O gen to silicon, silicon to iron. screaming outwards at a Si The star’s core starts to look few percent of the speed Fe like an onion, with layers of light: what astronomers nested inside one another. call a supernova. At every step, the process The seeds of a star’s ulti- generates more heat, and the mate destruction are plant- fusion goes ever faster. A star ed deep in its core, where its may fuse hydrogen into helium energy is generated. Stars are for millions or billions of years, giant balls of gas, and when a gas but by the time it starts to fuse sili- is compressed it heats up. Be- con into iron, it may take mere cause stars are so big they have Near the end of a massive star’s life, the fusion occurs in shells days. As iron piles up in the a lot of gravity, so at the core around the core, like the layers of an onion. core, the star is headed for di- of a star the pressure is intense. saster. This means they get very hot, hot enough to smash Why? Because up until iron, all the fusion reactions together atomic nuclei. And when nuclei collide, have produced energy in the form of heat. That heat they can stick together in a process called fusion.